The invention relates to the ionization of gaseous analyte molecules by physical or chemical reactions at atmospheric pressure (API) and the transfer of the analyte ions through an inlet capillary into the vacuum system of an ion spectrometer, for example a mass or a mobility spectrometer. The generation of ions of heavy analyte molecules with molecular weights of a few hundred to many thousand daltons in an electrospray ion source at atmospheric pressure is very well known. The ability to ionize macromolecules, which cannot be vaporized thermally, is extremely important; John Bennett Fenn was awarded with a part of the 2002 Nobel Prize for Chemistry for the development of the electrospray ion source toward the end of the 1980s.
In addition to electrospray ionization (ESI), which is mainly used for proteins and peptides, other types of atmospheric pressure ionization (API) have been developed: atmospheric pressure chemical ionization (APCI), atmospheric pressure photoionization (APPI), and atmospheric pressure laser ionization (APLI).
In the housing of an electrospray ion source, a high voltage of several kilovolts is applied to a pointed spray capillary containing spray liquid with dissolved analyte molecules: an extremely strong electric field is generated around the tip, and this field draws the spray liquid into a fine jet, which quickly disintegrates into minute, highly charged droplets with diameters in the order of a hundred nanometers to a few micrometers. The droplets then evaporate, leaving behind mainly multiply charged ions of the analyte molecules formerly contained in the droplets.
Since the droplets of the spray jet from the spray capillary are all very highly charged, they repel each other very strongly. This causes the spray mist to broaden into a pronounced funnel shape immediately after the droplets have been formed. A spray gas supplied in a sharply focused jet, which can be heated up to around 150° C., can be used to reduce the broadening of the spray mist. When spray gas is used, the analyte ions produced in the very elongated ion formation volume are usually extracted more or less perpendicularly by an electric drawing field and fed to the inlet capillary. This is successful for only a small portion of the analyte ions, however, because only analyte ions from a small section of the length and width of this ion formation volume reach the inlet capillary.
The ion source housing has a volume of around one liter and is somewhat irregularly shaped. Around the inlet capillary, further gas is blown into this ion source housing: the gas to transport the analyte ions through the inlet capillary. In the electrospray ion source housing, confused conditions therefore prevail, with sometimes turbulent gas flows (spray gas, transport gas) and intersecting electric fields (spray voltage, ion extraction voltage). This means it is difficult to guide the analyte ions through the turbulent gas flows to the tiny aperture of the inlet capillary; only very few of the analyte ions formed are actually guided to the inlet capillary.
In an APCI ion source for the chemical ionization of analyte substances at atmospheric pressure, the reactant ions are usually produced by a corona discharge at the tip of a tungsten pin. The reactant ions are usually generated from slightly moist nitrogen; a few nitrogen ions are produced initially, but these quickly react with water molecules and form different types of water complex ions, which can then react with analyte ions by protonation or deprotonation. These processes are known to the person skilled in the art. The analyte molecules are generated from a gas chromatograph or by the thermally assisted spraying of droplets in the spray gas with subsequent evaporation (“thermospray”). Current APCI ion sources are installed in housings which are similar to those of electrospray ion sources, so they can be easily exchanged with these, largely retaining the feed-ins for heated spray and transport gases and the voltage supplies. These housings are mostly unfavorable for introducing the analyte ions into the inlet capillary leading to the ion spectrometer because completely uncontrollable gas flows prevail inside them, including the strong wind produced by the corona discharge, and also largely uncontrollable electric fields caused, for example, by the electric field of the corona discharge and the discharge plasmas produced. Moreover, it is not possible to control how many of the analyte molecules are unintentionally decomposed by the corona discharge plasma.
The situation is similarly confused with current APPI ion sources. The photon impact ionization of these ion sources can act on the analyte molecules themselves, but usually other molecules are ionized by the photon impact, which then react with the analyte molecules in a chemical ionization. The analyte molecules can again originate from gas chromatographs or can be generated by the thermospraying of analyte solutions. The proportions of direct and indirect photoionization can hardly be controlled in a reproducible way. Confused gas flows also prevail in these ion source housings, and transport the analyte ions on sometimes wild trajectories before a very few of them reach the inlet capillary.
A relatively new ionization method is laser ionization at atmospheric pressure (APLI), in which analyte molecules, usually from gas chromatographs, are ionized by multi-photon processes in the beam of the UV light from a suitable pulsed laser. Even if the ionization is performed near the entrance aperture of the inlet capillary, not all analyte ions can be captured.
In an aspirating inlet capillary free-standing in a surrounding gas, a stably laminar flow is formed some distance behind the entrance aperture after some initial boundary turbulence. The boundary turbulence causes a loss of analyte ions. All these ion sources require that the analyte ions be introduced into this inlet capillary from an extensive ion formation volume, which is successful to only a very limited extent. Only if the analyte ions can successfully be introduced right into the laminar flow of the inlet capillary, a satisfactorily high proportion of these introduced analyte ions will be transported into the vacuum system of the ion spectrometer operating in vacuum.
The spectrometers here can be mass spectrometers or vacuum-operated mobility spectrometers, for example. The inlet capillary usually leads to a first stage of a differential pumping system. In this first stage of the vacuum system, the analyte ions can be captured by a so-called ion funnel, for example, separated from the accompanying gas and introduced into the ion spectrometer via further ion guides and pump stages. The analyte ions are then subjected to the desired type of analysis in the ion spectrometer.
When the term “atmospheric pressure” is used here, it should not be interpreted too narrowly. It is intended to include all pressures above approximately ten kilopascal, even if the term usually refers to ambient pressure.